According to German Pat. No. 1 004 516, the lens is for this purpose clamped at high pressure between two cups, so that its position cannot change by itself. To center the lens, the clamping cups are vibrated by ultrasound during the clamping operation in order to convert the static friction between cup and lens into a lower sliding friction. However, this transition occurred sporadically, which often caused damage to the lens due to an undesired material removal.
Furthermore, an attempt has been made to drive the clamping cups during clamping of the lens in opposite directions of rotation. Here too a high risk exists that lens damage will occur, that is, cutting tracks in the form of rings cut into the surface of the lens can hardly be avoided.
German Auslegeschrift No. 21 48 102 suggests to arrange a piezoceramic case vibrator on the elevationally nonchangeable clamping cup, which case vibrator is electrically controlled by a threshold switch such that the clamping cup force drops off when reaching a given pressure, which causes the vibration generator to be turned off. The piezovibrator is used at the same time to test the clamping pressure, to which the vibrational amplitude is regulated. Electronic instabilities are disadvantageous in this arrangement. Furthermore, the vibrator has a not insignificant sensitivity with respect to axial pressure. An initial stress is created during clamping due to the pressure load; a supporting of the vibrator is therefore problematic.
From German Offenlegungsschrift No. 31 39 873 a device is known in which the irregularities of a gear drive are utilized to produce relative movements between lens and clamping cup. A balanced differential is provided as a compensating device between the two parts of a two-part centering spindle and the drive shaft. A hydraulic clamping cylinder is provided for a pressure plate of the upper, axially movable spindle. Due to the high friction of the clamping spindle in its slide bearing, a precise regulating of the clamping pressure is, however, difficult to realize, so that this device can also only be utilized in a limited way.
The purpose of the invention is, while overcoming the disadvantages of the state of the art, to provide an improved drive for the centering spindle, to make possible, with a reliable clamping of the lens, a damage-free machining of the lens. Furthermore, the effective torque is most sensitively adjustable and regulatable at every instant.
By inventively drivingly connecting a drive shaft through a belt drive to the centering spindles, machining can be done at a high speed and correspondingly little surface removal at a high machining performance can be achieved at the periphery. The belt drive replaces a common gearing, so that gears with their rolling behavior, which is irregular in the micro-range, are excluded from the drive train for the centering spindles and their synchronous running is assured.
In addition it is possible that the drive shaft is drivingly connected through a further belt drive to the motor. It is further provided that the belt drive acts on the drive shaft through a torque divider. The torque divider is constructed in two parts with the torque divider being arranged in a conventional manner between its two parts. The drive shaft and/or the torque divider is supported in the housing by means of a bearing, which is preferably arranged at the height of the clamping cups parallel with respect to the direction of the axes. This permits a very precise adjustment of the clamping force and torque to the respectively given lens geometry. It is particularly advantageous that the bearing for the shaft parts on the output side is constructed smoothly and exactly fitting, in particular with precisely aligned radial bearings
The torque divider operates step-free and smoothly. It can have a guide member between two roller-supported plates, which are each drivingly connected to one of the shaft parts. This guide member can be constructed on the outer periphery as a belt pulley for the motor belt drive. This design is structurally simple; it assures a high efficiency of the drive with a degree of precision guiding.
An important further development of the invention consists in the guide member being a particularly plate-shaped ball race and the plate-shaped plate running with its peripheral parts on a ring of balls. Furthermore, the guide member can have shaft ends with shoulders, which are each supported through axial and radial bearings in the associated plates. Because of the balls rolling with relative movements along the friction plates, the frictional connection between the lens and the clamping cups overcomes the inner friction of the torque divider, so that an absolute synchronous run of the centering spindles is assured by the compensating movement.
It is advantageous if the guide member of the torque divider is rotatably supported with an adjustable or regulatable frictional connection between the plates, with at least one plate, being loadable axially toward the other plate by a pressure-medium-operated pressure piece. The clamping device can be operated through a pressure medium, which at the same time loads a piston for the pressure piece. The latter is guided axially movably in a plate and is preferably axially elastically supported, for example with a suspension.
Thus the torque divider consists of an upper and a lower plate. Its drive occurs steplessly and smoothly due to the balls being supported in the guide member, for which purpose the contact pressure adjusts the driving movement through the pressure piece onto the lower and upper plate.